1. Field of the Invention
The present invention relates to an electronic control unit for automobile and, more particularly, to a circuit of a power supply input section to a power supply control IC of the electronic control unit for automobile.
2. Description of the Related Art
In causing an electronic control unit for automobile (ECU) to operate, there is generally adopted a method of connecting a plus terminal of a battery and an ECU power supply input terminal and connecting a minus terminal of the battery to the GND to supply electric power to the ECU and cause the ECU to operate. However, it is likely that the terminals of the battery are reversely connected because of a human error. In the ECU in which measures against the reverse connection are not taken, an internal circuit is destroyed. Therefore, it is necessary to configure a reverse polarity protection circuit in an ECU power supply input section.
A method of connecting a diode to the power supply input section as the reverse polarity protection circuit has been used. If the battery is reversely connected, a path of an electric current flowing from a power supply control IC to the battery is cut off by a rectifying action of the diode. Therefore, a voltage applied, to the power supply control IC drops to nearly 0 V.
However, in this system, if the battery is normally connected, a voltage drop equivalent to a forward voltage of the diode occurs. Therefore, when a battery voltage drops to, for example, about an operable voltage of the ECU, it is likely that a voltage for the ECU to operate is insufficient and ECU reset occurs.
JP-A-7-244584 discloses a technology of a reverse polarity prevention circuit in which an MOSFET is used instead of the diode and a voltage drop does not occur.
In the reverse polarity protection circuit in the past in which the diode is used, when the battery voltage drops, it is likely that a voltage necessary for the operation of the power supply control IC cannot be supplied and the ECU malfunctions. There is no conception and disclosure concerning protection of an FET in JP-A-7-244584. Therefore, in the reverse polarity prevention circuit disclosed in JP-A-7-244584, when a surge occurs from an input terminal, for example, the FET is destroyed because of the surge occurrence and an abnormal current flows from a power supply.